The three main classic chronic myeloproliferative disorders recognized today are polycythemia vera, essential thrombocythemia and idiopathic myelofibrosis. A lack of reliable and readily available biomarkers for the chronic myeloproliferative disorders has led to diagnostic difficulties in the past. The diagnosis of both PV and ET has largely been by exclusion. However, recent reports described that 76% to 97% of patients with PV, 29% to 57% of patients with ET and 50% of patients with IMF have a somatic point mutation in JAK2. The mutation, JAK V617F, is not found in healthy subjects and is associated with constitutive phosphorylation of JAK2 and its downstream effectors. Laboratory evaluation of patients with suspected chronic myeloproliferative disorders includes molecular testing for the presence of the JAK2 mutation and immunophenotypic analysis of hematopoietic cells by flow cytometry. Morphologic review will be performed if material is available. We have developed in the past year novel RT-PCR/RFLP assay for detection of the JAK V617F mutation. We have also developed a flow cytometric panel for chronic myeloid disorders, which includes evaluation of CD177 expression on myeloid elements in patients with and without JAK2 V617F mutation. One of our goals is to investigate the possible role of neutrophil expression of CD177 gp as a biomarker of chronic myeloproliferative disorders. Changes in neutrophil expression of CD177 gp alone or in combination with other neutrophil membrane proteins might be useful biomarkers for the diagnosis or staging of ET or PV.